Any ideas on this odd electrical issue?

[The Goat]

Hello

I changed my day running light bulb the other day on my Volvo 740 (1989 B200E) after months of it being out of action. When I turned the car back on, the day running lights didn’t work nor did the dashboard lights.

Since then it hasn’t changed. When I turn on the lights the rear windscreen wiper goes on and the dashboard lights up. All fine if it’s raining otherwise my wiper will be worn out! When I turn the car off, the radio doesn’t turn off unless I switch it off manually (it’s an after market one which turned off automatically once the ignition was off). In addition the seatbelt warning light doesn’t turn off unless I turn the radio off. When I indicate left it also sets off the rear windscreen wiper for one wipe.

I have checked the wiring by the replaced light (by the air filter box) and looked for bare wires but can’t see any. I can’t see how changing one light can cause so many problems! Does anyone have any ideas?

Thanks in advance.